The official job posting doesn't mention a salary; neither does the informal posting. It acknowledges "Yes I know this is a hard-to-find (existing) skill." It doesn't say whether the compensation is commensurate. When pointy-haired bosses say that workers with certain skills are not available, what they often mean is that they are not available for the salary they feel like paying.
The PDP-11 was an extremely popular machine and remained so up to at least the mid-1980s. I remember the RSX people at DECUS griping about being high-hatted by the VAX people, and a slightly bitter funny song, "He's got the whole world on his VAX." Someone who was in their twenties at the height of the PDP-11's popularity would be in their fifties today.
And it's not an arcane architecture, it is crystal-clear, logical, and symmetrical; it set the pattern for a number of popular microprocessors, including the Motorola 68000, and the posting says plainly "We will also consider programming experience with other assembly language." Frankly, anyone with ANY experience in assembly language programming could pick it up in a couple of weeks.
I'm in my late sixties, retired, not interested in working full time or moving, but I can't believe their problem would be difficult to solve IF they offered appropriate compensation.